Chronic synchronized atrial-ventricular pacing has been shown to improve symptoms and effort tolerance in patients with obstructive hypertrophic cardiomyopathy, changes associated with reduction in the left ventricular outflow gradient. To assess the impact of atrial-ventricular pacing in nonobstructive hypertrophic cardiomyopathy, 7 patients underwent exercise hemodynamic, radionuclide angiographic, and thallium scintigraphic studies before and 1 to 6 months following implantation of a pacemaker programmed to synchronized atrial ventricular (DDD) pacing. All patients improved by at least one functional class and significantly increased treadmill exercise duration on the Standard Bruce Protocol by almost two and half minutes. During semi-erect bicycle exercise with DDD pacing, there was reduction in left ventricular filling pressures without compromise to cardiac output compared to the exercise study performed prior to pacemaker implantation. The rest and exercise left ventricular ejection fractions were unchanged by chronic pacing. Thallium scintigraphic evidence of myocardial ischemia during matched levels of exercise were improved or eliminated in 4 of 5 patients who underwent these studies. Thus, chronic atrial-synchronized ventricular (DDD) pacing benefits symptoms and exercise capacity of patients with nonobstructive hypertrophic cardiomyopathy, with reduction in exercise left ventricular filling pressures, preservation of rest and exercise systolic left ventricular function, and less ischemia during stress.